Joe E. Maskasky, "Epitaxial Selective Site Sensitization of Tabular Grain Emulsions", Journal of Imaging Science, Vol. 32, No. 4, July/August 1988, and Maskasky U.S. Pat. No. 4,435,501 (collectively referred to as Maskasky, except as noted) are cumulative reports of the first investigations of the selective siting of high chloride silver halide epitaxy on host high bromide {111} tabular grains. In most of the embodiments disclosed by Maskasky, the epitaxy is selectively directed to the edges of the host tabular grains, including direction in some instances exclusively to the comers of the host tabular grains.
The approach preferred by Maskasky and that subsequently has been adopted by others calls for precipitating a high bromide {111} tabular grain emulsion, washing the emulsion, and then adsorbing a spectral sensitizing dye onto the major faces of the tabular grains. When properly selected, spectral sensitizing dye directs epitaxy to the edges of the tabular grains during subsequent precipitation of high chloride silver halide. After epitaxy has been deposited, the emulsion is chemically sensitized, typically employing sulfur and gold sensitizers and holding the emulsion at an elevated temperature. Other descriptions of spectral sensitizing dye directed epitaxy are provided by Daubendick et al U.S. Pat. Nos. 5,494,789, 5,503,971 and 5,576,168, Deaton et al U.S. Pat. No. 5,582,965, Eshelman et al U.S. Pat. Nos. 5,612,175, 5,612,176 and 5,614,359, and Levy U.S. Pat. No. 5,612,177.
The use of spectral sensitizing dye as a site director for epitaxial deposition has a number of disadvantages. First, the host grains run the risk of being degraded prior to epitaxial deposition by removal from the reaction vessel, washing, and dye addition before commencing epitaxial deposition. Second, reliance on spectral sensitizing dye for cpitaxy siting restricts the possible choices of dyes. Third, the spectral sensitizing dye runs the risk of being degraded when heated to elevated temperatures during chemical sensitization.
Although not the preferred approach, Maskasky recognized that selective site epitaxial deposition onto to high bromide {111} tabular grains can be achieved without employing a spectral sensitizing dye. Maskasky recognized that host tabular grains containing greater than 8 mole percent iodide, based on silver, uniformly distributed, contain sufficient iodide to direct high chloride epitaxy in the absence of a spectral sensitizing dye. Unfortunately, this entails surface region iodide levels that are objectionably high for many imaging applications. Non-uniform iodide placements are shown to direct epitaxy, but a large proportion of the surface regions of host grains must exhibit high iodide levels to restrict epitaxy to a limited area of the host grains.
Maskasky (patent), column 65, Example 3B discloses an example of a host silver bromide {111} tabular grain emulsion having silver chloride epitaxy directed to the corners of the tabular grains using only a surface treatment of potassium iodide as a site director. In this example, the silver chloride epitaxy was introduced only after the emulsion had been precipitated and washed. In other words, epitaxy deposition was not conducted in the same reaction vessel as a continuation of the host grain precipitation.